A transformer, which is a stationary induction apparatus used in an electric power system or power reception and transformation is broadly divided into 1: a liquid cooled transformer that uses an insulating oil or liquid silicone, for example, 2: a gas insulated transformer whose insulation and cooling are based on inert gases such as SF6, and 3: a dry type transformer in which an iron core and windings are used in air. IEC (International Electrotechnical Commission) and JEC (Japanese Electrotechnical Committee of the Institute of Electrical Engineers of Japan), for example, which are applicable standards of a transformer, define a kind of dry type transformer in which the entire surface of windings are covered with resin or an insulating material including resin, as a molded transformer.
In recent years, transformers are increasingly required to be environmentally sustainable, noncombustible, and flame-retardant. For this reason, instead of a gas insulated transformer using inert gases such as SF6, which is a kind of a greenhouse gas, or a liquid cooled transformer requiring time and trouble for processing at a site, the demand for a dry type transformer is becoming higher. In particular, a molded transformer also uses a resin layer applied on windings for its insulating function, and can therefore have a higher insulating performance than other dry type transformers. Hence, molded transformers are increasingly used in particularly high-voltage fields.
However, insulation between a high-voltage winding and a low-voltage winding, and insulation between a member at a ground potential such as an iron core and a winding, are affected by air as well as the resin layer. Hence, application of conventional molded transformers have been limited to around 33 kV in Japan, and around 77 kV, except for in special cases, even in foreign countries such as Europe and the United States.
Also, since air at atmospheric pressure has higher viscosity and lower density than SF6 gas, for example, there is a limit to its cooling performance. For this reason, the transformer capacity of conventional molded transformers have been limited to about 15 MVA or lower.